[6] It is the principal parasite of the alkali bee Nomia melanderi, an important alfalfa pollinator across Washington, Oregon, Idaho, and Wyoming.

[6] Like the eggs of ground-nesting bees, they are laid by adults emerging in spring before the beginning of winter.

[6] They move by grabbing the surface with their mouth hooks and pushing themselves forward using their posterior prolegs.

[6] They dig to the surface and then immediately break out of the pupal skin,[6] which usually remains stuck to the ground and can sometimes be used to identify the presence of the parasites.

[7] Adults have unclouded wings, a tan thorax, and a gray and buff pattern on the abdomen, as well as a long proboscis.

[7] Older flies lose the majority of the hair on their abdomen and become uniformly dark gray.

[7] H. robustum is the principal parasite of the alkali bee Nomia melanderi, an important alfalfa pollinator (as of 1960) across Washington, Oregon, Idaho, and Wyoming, and can destroy large percentages of its larvae.